12V Battery near-dead after charging HV battery :-(

My 2012 PiP just started exhibiting some strange behavior with the 12V battery a few weeks ago, and I'm having trouble determining the actual cause. I haven't been able to determine yet if the battery needs replacement, or if the DC-DC converter isn't charging it properly. Several times the car has either been unable to start, or has just barely been able to get enough power to start from the 12V battery. I took it to the dealership after the second time this happened, and they checked the 12V battery and of course said "no problem found". Then it's happened twice more since and I started investigating further on my own.

I started to notice a pattern that it's always been at a time that is just after a charging session, coupled with needing to start the car twice (i.e. once to move the car in the driveway, then a second time later on to drive for real -- thus putting a bit more of a burden on the 12V battery, but nothing unreasonable). Normally, I use an L2 station at home, and sometimes an L1 charge when away from home. The 12V problem has been much more prevalent right after we've done an L1 charge (i.e. when it takes twice as long). This led me to the idea that maybe the 12V battery just is getting drained. My understanding is that the 12V battery should be maintained/recharged whenever the HV battery is being charged from the wall. During this time, the DC-DC converter should be running and keep the 12V system alive and well, since the computers need power during the charging session, and you don't want to drain the 12V battery every time. Thus, if it's not charging it properly, it will be much worse off on a 3-hour L1 charge than a 1.5-hour L2 charge.

So, here's the results of a more controlled experiment I did last night, purposefully doing an L1 charge at home and measuring the 12V battery at various times. This resulted in perfectly reproducing the PiP being unable to start, as well, so I definitely nailed the steps to reproduce (which of course the dealership didn't know when I first took it there).
1) PiP comes home for the day, had just been out on a long drive, so 12V battery has been topped off by car running normally. HV battery is near-empty, ready for a charge. 12V battery measures a healthy 12.4V with the car off.
2) Plug in car to L1 EVSE. Battery now reads 12.1V.
3) 90 minutes of charging later, battery reads 11.6V.
4) After 120 mins: still 11.6V
5) After 150 mins, it had finished HV charging: 12.0V
6) Turned car to ig-on mode: Battery immediately falls to 10V and falling below that fast (dropping 0.1V every second!). I turn it off in about 10 seconds, after it's around 9.0V.
7) A few seconds after PiP turned off: rebounds to 11.0V
8) Tried to start to Ready mode and failed (dashboard flickers, won't start). Turned it off immediately, battery reads 9.7V.
9) Plugged in L1 EVSE (thankfully enough 12V power to start the charging process!), slooooowly went up to 11.6V, then 12.0V over the next minute or so.
10) Charging stopped after 2-3 mins (since it didn't really need to charge the HV battery more). Now the 12V battery reads 10.5V.
11) Plug in 12V battery tender and go to bed

So, the obvious thought is the 12V battery isn't too healthy and maybe needs replacement. BUT:
- The dealership tested it and said it was fine. (Need to take it somewhere else for a second opinion)
- We were just out of town for a week, and when we came back the PiP started fine. It doesn't seem to lose charge if it's just sitting there.
- The car is only 2.5 years old! I know that 12V battery issues are "known" in the Prius, but this seems way too early for that, and my 2006 Prius never needed 12V battery help for the 7+ years I owned it.

So what's the deal with the DC-DC converter?
- When the car is in Ready mode, the 12V system is at a completely normal 14-14.2V, indicating that it's charging the 12V battery as expected.
- Why does the 12V system seem to want to go to 11.6V when the HV system is charging? Shouldn't it be at 14V then as well? Seems like it's actually DIScharging my 12V battery during that time!
- It doesn't seem completely failed since on my second plug-in attempt it did at least go from 9.7V to 11.6V. Just seems way too low of a voltage to actually charge the battery.

I don't really want to take it back to the dealership until I am reasonably confident that it has a DC-DC converter problem. If it's really just the 12V battery, that's not under warranty, and I don't need to pay them $300+ to replace that (or even just an hour of "wasted" labor for them to come to that conclusion). They already wasted my time once.

Does anyone know more about how the DC-DC converter should be charging the 12V battery while the car is plugged in and charging the HV battery? Can someone else go measure their 12V system on their PiP while charging? It would be most helpful to know if another car is at the same 11.6V as mine is during that time, or the more "expected" 14V.

The 12V battery probably remains the more likely culprit, but I'd like to make a more educated decision on what to do next. I'd hate to start burning through 12V batteries because the car just isn't keeping it maintained properly.

Thanks for reading this far -- I wanted to capture as much info as possible. We really need to fix this ASAP, as right now we are worried about the car being able to start every time we need to drive it!

 

As stated: "When the car is in Ready mode, the 12V system is at a completely normal 14-14.2V"
I only question it because I would have expected the same 14V when it is in HV-charging mode. But, it could be completely different and I am just making a poor assumption .

Thanks for the tip, a good thing to try.

 

Nominal voltage would be 12.6V, right? I figure that's close enough, especially since I'm measuring from the battery terminal in the front fuse box instead of at the battery itself.

 

Dealer wants $190 for the battery at the parts department. The Optima YellowTop battery is $210 locally and only as low as about $150 online. I sure wish it were only $100!

But you are absolutely right that it would prove it one way or the other...

 

Yeah, it is kind of ridiculous. My Leaf will automatically charge the 12V battery from the HV battery every 5 days if the car isn't started at all (though with the strange caveat that it also has to be UNplugged... I found that one out the hard way!)

There is a Batteries Plus nearby, which has the Optima YellowTop, as well as a slightly cheaper Duracell battery. But at $210 and $180, respectively, I might as well just get the OEM battery with a better warranty at the dealership for $190. I have been starting to wonder about the history of the 12V battery in here... my PiP was a dealer demo car that I bought CPO from them. I got the feeling from the car history that it hadn't been driven very much in the year preceding my purchase, which suggests the 12V battery wouldn't have been in the best care. But odd to not have it show any effects for more than a year after I bought it. The car is often not driven for a week or two at a time and always starts normally after that (except once last winter when it sat over 2 weeks with my OBD dongle plugged in... I'll take the blame for that one though ), so I've never suspected any issues with the 12V battery.

 

I'm not 100% convinced the DC-DC converter is completely off, because the second time I plugged in (see step #9), it went from 9.7V up to 11.6V, and then 12.0V a little later. Something has to be putting that voltage out there, and that something has to be either a DC-DC converter from the HV power bus or an AC-DC converter from the J1772 input. It's just not enough to actually charge the 12V battery properly, which is what has been making me question it.

I did also consider the possibility that there's an extra parasitic load. I'm sure a locked fan rotor would throw a code. Originally I thought that my OBD reader had maybe become faulty and was loading down the 12V system, but I unplugged it entirely and it made no difference. I could put a current clamp on the 12V system to monitor it, but without another known-good PiP next to it to compare, it's practically a worthless test.

 

I took the battery tender off the 12V battery this afternoon. It was at 12.9V at that time, and fell to 12.7V over the next 5 minutes. 6 hours later, it is still at 12.6V. So no, it has not seen the same voltage drop as it does while the car is charging.